The Department of Health, doctors’ and nurses’ unions and the contractors paid to carry out the assessments have refused to pledge to take any action following new research linking the government’s “fitness for work” test with about 600 suicides.

The study funded by the National Institute for Health Research – which itself is funded by the Department of Health – concluded that the programme to reassess people on incapacity benefit (IB) using the work capability assessment (WCA) could have caused 590 suicides in just three years.

Public health experts from Liverpool and Oxford Universities show in the study that across England as a whole, the reassessment process from 2010 to 2013 was “associated with” an extra 590 suicides, 279,000 additional cases of self-reported mental health problems, and the prescribing of a further 725,000 anti-depressants.

The researchers warn health professionals such as doctors and nurses that their involvement in the WCA process as assessors employed by the US outsourcing giant Maximus “raises major ethical issues”.

But despite that warning, the nurses’ union, the Royal College of Nursing (RCN), appeared to dismiss the need to take any action.

An RCN spokesman said: “As with nursing staff in many fields of practice, these professionals may come up against ethical dilemmas.

“The NMC’s code for nurses and midwives is well placed to help deal with these dilemmas, particularly the first theme, which is prioritise people.

“Nursing staff know the importance of acting within the code and it is used for professional guidance across all work settings.”

But he refused to say if the union would take any action, such as offering guidance or advice, or even alerting members to the research.

The British Medical Association, the doctors’ trade union, was too busy to comment on the research, and the researchers’ warning, or the 590 suicides.

And a spokesman for Maximus, which is being paid £595 million to carry out the WCA on behalf of the government, said: “That research you have referenced does not mention us so we have no comment to make on it.”

The Department of Health has also declined to say whether it will take any action, or even to comment on the findings, pointing only to a statement by the Department for Work and Pensions (DWP).

A DWP spokesman said in its statement: “This report is wholly misleading, and the authors themselves caution that no conclusions can be drawn about cause and effect.

“In addition, it is concerning that they provide no evidence that the people with mental health problems highlighted in the report even underwent a work capability assessment.”

He pointed to the five independent reviews of the WCA – the first three carried out by Professor Malcolm Harrington – and “significant improvements to the process” made by DWP since 2010, including how the WCA assesses people with mental health conditions for their eligibility for employment and support allowance (ESA).

He also said the percentage of people with mental health conditions who receive the highest level of ESA support has “more than tripled since 2010”.

But he refused to comment on whether DWP agreed that the involvement of healthcare professionals in the assessment regime now “raises major ethical issues for those involved”.

Disability News Service reported earlier this month how work and pensions ministers Chris Grayling and Iain Duncan Smith had failed to pass on a letter to Harrington that had been written by coroner Tom Osborne in the wake of the suicide of a disabled man, Stephen Carre (pictured), in 2010.

The letter said Carre’s death had been triggered by being found “fit for work”, and it called for a review of the policy not to seek further medical evidence from a GP or psychiatrist if a claimant had a mental health condition.

But Grayling and Duncan Smith decided to roll out the WCA to existing IB claimants, despite the letter from the coroner, while they also appear to have failed to complete a substantive response to that letter.

More than three years after Osborne sent his report to ministers, another coroner wrote an almost identical letter warning of similar concerns about the safety of the WCA, this time after the death of a north London man, Michael O’Sullivan.

When asked whether Grayling – now leader of the House of Commons – and Duncan Smith would now resign, in the light of these revelations and the new research, the DWP spokesman refused to comment.

The study’s lead author, Ben Barr, senior clinical lecturer in applied public health research at the University of Liverpool, said DWP’s response to the study was “disappointing”, and that none of the other factors researchers had looked at as possible causes, such as cuts to local government services or a fall in wages, could explain the rise in suicides and the deterioration in mental health.

He said these increases only happened among age groups most affected by the WCA, while the rise in mental health problems “tended to occur shortly after the increase in people undergoing the WCA in each area”.

Barr also called on DWP to publish any data it has on claimants’ mental health before, during and after the assessment.

He said: “Unfortunately, the DWP implemented the policy without a controlled trial or any plans to evaluate its impact on mental health.

“Given that data is not currently available on the specific individuals who underwent the WCA and there is no trial evidence, the next best approach to investigate the potential effects on mental health is the one we applied in our study, using appropriate statistical methods to control for alternative explanations for these trends.

“Our findings should at the very least raise serious concerns for the DWP that the potential negative impacts of the WCA need to be investigated further.”